Noise filter and noise filter array

ABSTRACT

A noise filter includes a plurality of LC parallel resonant circuits having a plurality of coils which are connected in series and electrically connected to external electrodes at both ends thereof, and capacitors which are connected in parallel to the coils, respectively, and are disposed inside an insulator while being sequentially connected in tandem to signal wires. Resonance frequencies of the respective LC parallel resonant circuits are preferably different from each other. Further, a shield electrode is disposed between the coils, and the shield electrode also defines a capacitance-forming electrode for preventing magnetic coupling between the two coils.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a noise filter for effectively removingnoise flowing in a signal wire disposed on a circuit board, and a noisefilter array.

2. Description of the Related Art

Depending on the communication system of a portable telephone, forexample, a single portable telephone may use a plurality ofcommunication bands. In order to prevent degradation of receptionsensitivity in each of the communication bands, it is necessary toeffectively remove noise in each frequency band.

Known examples of a noise filter used for such noise removal include achoke coil, a ferrite bead, and a ladder-type LC filter.

When the choke coil mentioned above is used as the noise filter, noisecountermeasures can be easily implemented because noise can be removedby simply connecting the choke coil to each signal wire. However, thechoke coil can only remove noise at a specific frequency because theband for which noise removal can be performed is relatively narrow,which disadvantageously makes it difficult to remove noises in aplurality of frequency bands at the same time.

Further, when a ferrite bead is used, as in the case of the choke coil,noise countermeasures can be easily implemented because noise can beremoved by simply connecting the ferrite bead to each signal wire.However, since a ferrite bead removes noise even in a low frequencyband, it exerts a large influence on the signal waveform such as byattenuating a necessary signal. Further, since high attenuation cannotbe attained, it may be impossible to achieve a satisfactory noiseremoval effect.

Further, the ladder type LC filter mentioned above comes in varioustypes, such as a T type, a π type, or an L type. Although any one of theabove-described types of ladder type LC filter can provide wide-bandnoise removal characteristics through appropriate setting of theinductance and capacitance, since it is necessary to ground an externalelectrode connected to a capacitor, it is essential to form a groundingelectrode pattern on a circuit board to which the ladder type LC filteris mounted. This disadvantageously limits the freedom of wiring layouton the circuit board.

Further, while a plurality of signal wires are formed on a circuit boardinvolving high-density mounting, depending on the component layout, itmay be difficult to form grounding electrode patterns having asufficient line width together with these signal wires. As a result, dueto the influence of a parasitic inductance in the grounding electrodepatterns, the frequency characteristics of the ladder type LC filterchange, which disadvantageously makes it impossible to remove noise in asatisfactory manner.

On the other hand, in the related art, there has been proposed a noisefilter constructed as follows (see, for example, Japanese UnexaminedPatent Application Publication No. 5-267059). That is, the noise filterincludes a filter element composed of one trap circuit formed by theinductance of a coil, which is composed of a plurality of coilconductors laminated in a spiral fashion within a dielectric, and afloating capacitance between the coil conductors. On either side of thiselement, a filter element composed of one trap circuit formed by theinductance of a coil, which is composed of a plurality of coilconductors laminated in a spiral fashion within a magnetic material, anda floating capacitance between the coil conductors, is arranged, andthese filter elements are integrated with each other to thereby form thenoise filter.

According to this noise filter, the resonance frequency of the trapcircuit constituting each of the filter elements is set to correspondwith each of a plurality of communication bands, thereby making itpossible to remove noise in each of the communication bands.

However, in the noise filter described in Japanese Unexamined PatentApplication Publication No. 5-267059, since not only the resonancefrequency on the high frequency side but also that on the low frequencyside is dependent on the floating capacitance generated between the coilconductors, it is not always easy to perform noise removal in anappropriate and satisfactory manner for each frequency band.

That is, in an LC parallel resonant circuit, the resonance frequency isdependent on the value of the LC product; the larger the LC product, thesmaller the resonance frequency. Here, the setting of the resonancefrequency on the high frequency side can be readily realized byadjusting the floating capacitance because the LC product may be set tobe small. On the other hand, for the setting of the resonance frequencyon the low frequency side, the LC product must be set to be relativelylarge. In this case, since problems such as distortion of the signalwaveform occur when the value of the inductance L is set too large,there is naturally a limit as to how large the value of the inductance Lcan be set. Therefore, in order to compensate for the shortage of theinductance L, it is necessary to obtain a relatively large floatingcapacitance by reducing the inter-layer distance between the coilconductors or by changing the insulation material.

However, when the inter-layer distance between the coil conductors isreduced as described above, this causes deterioration in characteristicsor reliability. Further, in the case where the insulation material ischanged, there is a problem in that the number of manufacturingman-hours increases due to the occurrence of delamination depending onthe characteristics of the material or due to an increase in the kindsof sheets to be used. In the case of Japanese Unexamined PatentApplication Publication No. 5-267059, in particular, because it isnecessary to fire the dielectric and the magnetic material at the sametime for integration, there is a problem in that not only is thereliability in terms of strength low due to cracks, peels, or the likethat are liable to occur during the manufacturing process, but also anincrease in cost is caused due to the necessity of setting and managingthe optimum manufacturing conditions with high precision.

Further, in the related art, there has been also proposed a constructionin which a plurality of coils each composed of a plurality of coilconductors laminated in a spiral fashion within a single dielectric areformed at the same time to thereby form a plurality of trap circuits.

However, as in the case of Japanese Unexamined Patent ApplicationPublication No. 5-267059 described above, also in the case of the noisefilter of this construction, it is difficult to set each trap circuit toa desired frequency in correspondence with a plurality of communicationbands, and further, there maybe cases where the coils tend to readilymagnetically couple with each other. Thus, a plurality of trap circuitscannot be formed or high attenuation cannot be attained at the resonancefrequency of each trap circuit, which disadvantageously makes itimpossible to perform noise removal in an appropriate and satisfactorymanner for each frequency band.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a noise filter which makes it possibleto easily and reliably set a resonance frequency in each of a pluralityof frequency bands and which is capable of efficiently removing noise ineach of the plurality of frequency bands, and also, a noise filter whichmakes it possible to attain a high attenuation at each resonancefrequency by reliably preventing magnetic coupling between the coils,and a noise filter array including such a noise filter.

According to a preferred embodiment of the present invention, a noisefilter for removing noise flowing in a signal wire located on a circuitboard includes a pair of external electrodes that are connected to thesignal wire and disposed on an outside of an insulator, and on an insideof the insulator, a plurality of coils are connected in series and haveboth ends thereof electrically connected to the external electrodes,respectively, and a capacitor is connected in parallel to at least oneof the plurality of coils, the coils each being defined by a pluralityof coil conductors which are laminated through the insulator andconnected to each other in a spiral configuration through a via hole,and the capacitor is defined by a shield electrode and acapacitance-forming electrode arranged so as to be opposed to each otherthrough the insulator, the shield electrode being located betweenupstream and downstream coils and commonly electrically connected toboth the upstream and downstream coils, the capacitance-formingelectrode being electrically connected to one of the pair of externalelectrodes.

According to another preferred embodiment of the present invention, anoise filter for removing noise flowing in a signal wire located on acircuit board includes a pair of external electrodes that are connectedto the signal wire and that are located on an outside of an insulator,and on an inside of the insulator, a plurality of coils are connected inseries and have both ends thereof electrically connected to the externalelectrodes, respectively, and a capacitor is connected in parallel to atleast one of the plurality of coils, the coils are each defined by aplurality of coil conductors which are laminated through the insulatorand connected to each other in a spiral configuration through a viahole, and the capacitor is defined by the coil conductors and acapacitance-forming electrode arranged so as to be opposed to each otherthrough the insulator, the capacitance-forming electrode beingelectrically connected to one of the pair of external electrodes.

In this preferred embodiment of the present invention, a shieldelectrode is preferably disposed between upstream and downstream coilsso as to be substantially perpendicular to a coil axis direction.

Further, according to another preferred embodiment of the presentinvention, a noise filter for removing noise flowing in a signal wirelocated on a circuit board includes a pair of external electrodes thatare connected to the signal wire and are located on an outside of aninsulator, and on an inside of the insulator, a plurality of coils areconnected in series and have both ends thereof electrically connected tothe external electrodes, respectively, and a capacitor is connected inparallel to at least one of the plurality of coils, the coils are eachdefined by a plurality of coil conductors which are laminated throughthe insulator and connected to each other in a spiral configurationthrough a via hole, and the capacitor is defined by the pair of externalelectrodes and a shield electrode arranged so as to be opposed to eachother through the insulator, the shield electrode being located betweenupstream and downstream coils and electrically connected to both theupstream and downstream coils.

It is preferred that the shield electrode is arranged so as to have asurface area that is equal to or larger than about ½ of a surface areaof a bore of at least one of the upstream and downstream coils.

It is also preferable that a plurality of LC parallel resonant circuitshaving different respective resonance frequencies are defined by thecoils and the capacitor which is individually connected in parallel toeach of the coils.

It is also preferred that an LC parallel resonant circuit on a lowfrequency side is defined by the coil, the capacitor connected inparallel to the coil, and a floating capacitor generated due to thepresence of the coil, and an LC parallel resonant circuit on a highfrequency side is defined by the coil and a floating capacitor generateddue to the presence of the coil.

According to another preferred embodiment of the present invention, anoise filter array includes a plurality of the noise filters accordingto any of the above-described preferred embodiments that are integratedtogether while being arranged in an array individually in correspondencewith a plurality of signal wires located on a circuit board.

In such a construction, connecting points between the coils provided toeach of the signal wires are preferably commonly connected together inan ungrounded state via a noise dispersing capacitor.

In the noise filter according to a preferred embodiment of the presentinvention, the plurality of LC parallel resonant circuits are disposedwithin the insulator while being sequentially connected in series to thesignal wire. Accordingly, by setting the resonance frequencies of therespective LC parallel resonant circuits to be different from eachother, it is possible to effectively remove noise for each of theplurality of frequency bands.

Accordingly, by using the noise filter according to various preferredembodiments of the present invention, noise countermeasures for portabletelephones, for example, can be effectively implemented.

Further, the coils used are preferably of a laminated type, and thecapacitor is defined by the shield electrode being located between theupstream and downstream coils and commonly electrically connected toboth the coils, and one of the two external electrodes so as to beopposed to each other through the insulator. Accordingly, the coil andcapacitor for defining each LC parallel resonant circuit can have arelatively simple construction. Further, a desired resonance frequencycan be readily set by adjusting the capacitance of the capacitor,varying the numbers of turns of the coil conductors, or varying thedistance between the coil conductors.

Further, since the LC parallel resonant circuits each including the coiland capacitor are disposed within a single insulator, it is possible toprovide a highly reliable noise filter array that is free fromstructural flaws with relatively little fear of cracks or peelsoccurring during the manufacture thereof.

Further, the plurality of LC parallel resonant circuits can be formed tohave a relatively simple construction also in the case where, as invarious preferred embodiments of the present invention, a laminated typecoil is preferably used as the coil, and the capacitor is constructed byarranging the coil conductor and the capacitance-forming electrode,which is electrically connected to one of the two external electrodes,so as to be opposed to each other through the insulator. Further, adesired resonance frequency can be readily set by adjusting thecapacitance of the capacitor, varying the numbers of turns of the coilconductors, or varying the distance between the coil conductors.Accordingly, noise can be effectively removed for each frequency band.

Further, when the shield electrode is disposed between the upstream anddownstream coils so as to be substantially perpendicular to the coilaxis direction, magnetic coupling between the upstream and downstreamcoils can be reliably prevented, whereby the setting of the resonancefrequency for each LC parallel resonant circuit can be performed withreliability.

Further, the plurality of LC parallel resonant circuits can be formed tohave a relatively simple construction also in the case where, alaminated type coil is used as the coil, and the capacitor isconstructed by arranging one of the two external electrodes and theshield electrode, which is located between the upstream and downstreamcoils and commonly electrically connected to both the coils, so as to beopposed to each other through the insulator. Further, a desiredresonance frequency can be readily set by adjusting the capacitance ofthe capacitor, varying the numbers of turns of the coil conductors, orvarying the distance between the coil conductors. Therefore, noise canbe effectively removed for each frequency band.

When the shield electrode is set so as to have a surface area that isequal to or larger than about ½ of the bore of at least one of theupstream and downstream coils, the magnetic coupling between theupstream and downstream coils can be prevented even more reliably.

Accordingly, the trap attenuation in each of the plurality of frequencybands can be very large without varying the resonance frequencies of therespective LC parallel resonant circuits. As a result, noise can beremoved even more effectively.

When the plurality of LC parallel resonant circuit shaving respectiveresonance frequencies that differ from each other are defined by thecoils and the capacitor individually connected in parallel to each ofthe coils, the resonance frequency of each of the LC parallel resonantcircuits can be reliably and readily adjusted or controlled to a desiredfrequency by appropriately setting the inductance of the coil and thecapacitance of the capacitor. Accordingly, noise removal can beperformed in a satisfactory manner for each frequency band.

Also in the case where the LC parallel resonant circuit on the lowfrequency side is defined by the coil, the capacitor connected inparallel to the coil, and the floating capacitor generated due to thepresence of the coil, and the LC parallel resonant circuit on the highfrequency side is defined by the coil and the floating capacitorgenerated due to the presence of the coil, noise removal can beperformed in a satisfactory manner for each frequency band despite theeven more simplified construction. That is, with respect to the LCparallel resonant circuit on the low frequency side, a somewhat large LCproduct can be set by the coil and the capacitor, whereby noise on thelow frequency side can be removed in a satisfactory manner. Further,with respect to the LC resonant circuit on the high frequency side,since the LC product may be set smaller than that on the low frequencyside, noise on the high frequency side can be removed in a satisfactorymanner by adjusting the coil and the floating capacitor generated due tothe coil.

Further, in the noise filter array according to a preferred embodimentof the present invention, a plurality of the noise filters according toany of the preferred embodiments of the present invention describedabove are integrated together while being arranged in an arrayindividually in correspondence with the plurality of signal wiresdisposed on the circuit board. Accordingly, noise in each of theplurality of signal wires can be removed by a single component (an arraynoise filter). Accordingly, it is not necessary to provide noise filtersindividually in correspondence with the respective signal wires, wherebyin addition to a reduction in the number of components as compared withthe prior art, it is possible to achieve an improvement in theefficiency of mounting of components and a reduction in the mountingsurface area on the circuit board.

When the connecting points between the coils provided for each of thesignal wires are commonly connected together in an ungrounded state viathe noise dispersing capacitor, a high attenuation can be attained ascompared with the case where no noise dispersing capacitor is provided,and further, the noise filter array obtained has a sharp cut offcharacteristic, thereby making it possible to suppress the influenceexerted on the signal waveform. Further, since it is not necessary toform the grounding electrode pattern on the circuit board, it ispossible to enhance the freedom of wiring layout on the circuit board,and to obviate the need to provide a large-area grounding electrodepattern in the inner portion of the circuit board. It thus becomespossible to achieve a reduction in the cost of the circuit board.

It is known that capacitive coupling between a plurality of signal wirescauses problems such as cross talk. Further, it is known that generally,the signal frequency is not higher than several tens MHz and the noisefrequency is within the GHz band. It is thus important to set the valueof the noise dispersing capacitor (particularly, to use a capacitorhaving a small capacitance value) such that the influence of cross talkdoes not appear in the waveform. By setting the value of the noisedispersing capacitor to an appropriate value, it is possible to disperseonly noise current to another signal wire.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a noise filter array according to a firstpreferred embodiment of the present invention as mounted on a circuitboard.

FIG. 2 is a sectional view taken along the line A-A of FIG. 1.

FIG. 3 is an equivalent circuit diagram of the noise filter arrayaccording to a preferred embodiment of the present invention.

FIG. 4 is an exploded perspective view showing a method of manufacturingthe noise filter array according to a preferred embodiment of thepresent invention.

FIG. 5 is a characteristic chart showing the frequency-dependentcharacteristic of insertion loss in the noise filter array according toa preferred embodiment of the present invention.

FIG. 6 is a sectional view showing the construction of a noise filterarray according to another preferred embodiment of the presentinvention.

FIG. 7 is a sectional view showing the construction of a noise filterarray according to a further preferred embodiment of the presentinvention.

FIG. 8 is a sectional view showing the construction of a noise filterarray according to a preferred embodiment of the present invention.

FIG. 9 is an equivalent circuit diagram of the noise filter arrayaccording to a preferred embodiment of the present invention.

FIG. 10 is an exploded perspective view showing a method ofmanufacturing the noise filter array according to a preferred embodimentof the present invention.

FIG. 11 is an equivalent circuit diagram showing a modification of thenoise filter array according to a preferred embodiment of the presentinvention.

FIG. 12 is an equivalent circuit diagram showing the wiring state whenan evaluation experiment is performed on the noise filter arrayaccording to a preferred embodiment of the present invention.

FIG. 13 is a characteristic chart showing the frequency-dependentcharacteristic of insertion loss in the noise filter array according toone preferred embodiment of the present invention as compared with thecase of another preferred embodiment of the present invention.

FIG. 14 is a characteristic chart showing the frequency-dependentcharacteristic of insertion loss when the capacitance of a noisedispersing capacitor is varied in the noise filter array according to apreferred embodiment of the present invention.

FIG. 15 is a characteristic chart showing the results of measurement ofthe frequency-dependent characteristic of insertion loss in the case ofthe construction shown in FIG. 9 and that in the case of theconstruction shown in FIG. 11, in the noise filter array according to apreferred embodiment of the present invention.

FIG. 16 is a sectional view showing the construction of a noise filterarray according to a preferred embodiment of the present invention.

FIG. 17 is an equivalent circuit diagram of the noise filter arrayaccording to a preferred embodiment of the present invention.

FIG. 18 is an exploded perspective view showing a method ofmanufacturing the noise filter array according to a preferred embodimentof the present invention.

FIG. 19 is a sectional view showing the construction of a noise filterarray according to a preferred embodiment of the present invention.

FIG. 20 is a sectional view showing the construction of a noise filterarray according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Now, characteristic features of the present invention will be describedin detail with reference to preferred embodiments thereof.

First Preferred Embodiment

FIG. 1 is a plan view showing a noise filter array according to a firstpreferred embodiment of the present invention as mounted on a circuitboard, FIG. 2 is a sectional view taken along the line A-A of FIG. 1,FIG. 3 is an equivalent circuit diagram of the noise filter arrayaccording to the first preferred embodiment of the present invention,and FIG. 4 is an exploded perspective view showing a method ofmanufacturing the noise filter array according to the first preferredembodiment 1 of the present invention.

As shown in FIGS. 1 to 3, the noise filter according to first preferredembodiment serves to remove noise flowing in a plurality of (in thepresent preferred embodiment, four) signal wires 2 located on a circuitboard 1. Four noise filters 3 are each preferably integral with arespective one of the signal wires 2.

That is, this noise filter array includes a substantially rectangularinsulator 4 formed by laminating and then integrally firing insulatingsheets such as ceramic green sheets. Further, on both end sides (outerleft and right portions) of the insulator 4, external electrodes 6, 7for signal input/output are formed individually in correspondence withthe respective signal wires 2. The external electrodes 6, 7 areelectrically connected by soldering or the like to left and rightelectrode patterns 2 a, 2 b constituting the respective signal wires 2,respectively.

Further, inside the insulator 4, two upstream and downstream LC parallelresonant circuits 8, 9 are arranged so as to be connected in tandem incorrespondence with the respective signal wires 2. The two LC parallelresonant circuits 8, 9 constitute the noise filter 3 with respect toeach of the signal wires 2.

As will be described later, the respective resonance frequencies of theLC parallel resonant circuits 8, 9 are preferably different from eachother so that noise can be effectively removed in each of a plurality offrequency bands.

Here, the upstream LC parallel resonant circuit 8 includes an input-sidecoil 11 and an input-side capacitor 12 connected in parallel with theinput-side coil 11, and the downstream LC parallel resonant circuit 9includes an output-side coil 13 and an output-side capacitor 14connected in parallel with the output-side coil 13. In the presentpreferred embodiment, the input-side coil 11 on the upper side and theoutput coil 13 on the lower side will hereinafter be also referred to asthe “upstream and downstream coils 11, 13”. The upstream and downstreamcoils 11, 13 correspond to the “upstream and downstream coils” recitedin the claims.

The input-side coil 11 is preferably formed as a spiral coil bysequentially connecting together a plurality of coil conductors 16,which are laminated inside the insulator 4, through a via hole 17.Likewise, the output-side coil 13 is preferably formed as a spiral coilby sequentially connecting together a plurality of coil conductors 18,which are laminated inside the insulator 4, through a via hole 19. Inthis preferred embodiment, the number of turns is set to be differentbetween the input-side coil 11 and the output-side coil 13 so that theresonance points of the respective LC parallel resonant circuits 8, 9are different from each other.

Further, first ends of the input-side coil 11 and output coil 13 areconnected to each other in series through a via hole 20, and second endsof the input coil 11 and output coil 13 are connected to the externalelectrodes 6, 7 on the input side and the output side, respectively.

Further, a shield electrode 23 is arranged between the upstream anddownstream coils (the input coil 11 and the output coil 13) so as to besubstantially perpendicular to the coil axis direction. Two upper andlower capacitance-forming electrodes 24, 25 are arranged so as to beopposed to the shield electrode 23 through the insulator 4. The shieldelectrode 23 and the upper capacitance-forming electrode 24 constitutethe input-side capacitor 12, and the shield electrode 23 and the lowercapacitance-forming electrode 25 constitute the output-side capacitor14.

Further, the shield electrode 23 is electrically connected to the viahole 20 that provides serial connection between the input-side coil 11on the upper side and the output-side coil 13 on the lower side, and isembedded in the insulator 4 so as to allow no external connection.Further, the shield electrode 23 preferably has a surface area that islarge enough to cover the bore of the upstream and downstream coils 11,13.

That is, the shield electrode 23 can function as one electrode forforming the capacitance of each of the capacitors 12, 14 as well as anelectromagnetic shield for preventing electromagnetic coupling betweenthe upstream and downstream coils 11, 13.

From the viewpoint of preventing the electromagnetic coupling betweenthe upstream and downstream coils 11, 13, it is preferable that thesurface area of the shield electrode 23 is equal to or larger than about½ of the surface area of the bore of at least one of the upstream anddownstream coils 11, 13.

Further, first end portions of the respective capacitance-formingelectrodes 24, 25 are led out to the outer side portions of theinsulator 4 to be electrically connected to the external electrodes 6,7, respectively. Further, by previously adjusting the surface area orthe distance over which the shield electrode 23 and each of the upperand lower capacitance-forming electrodes 24, 25 are opposed to eachother in order to thereby vary the capacitances of the input-sidecapacitor 12 and output-side capacitor 14, the resonance points of therespective LC parallel resonant circuits 8, 9 are adjusted so that anoise having a frequency to be removed can be removed in a satisfactorymanner. The resonance points can be also adjusted by adjusting theinductance of each of the individual coils 11, 13.

Next, a method of manufacturing the noise filter array according to thefirst preferred embodiment of the present invention will be described.

To manufacture the noise filter array according to the first preferredembodiment, as shown in, for example, FIG. 4, a predetermined number ofinput-side-coil-forming insulating sheets 31, output-side-coil-forminginsulating sheets 32, capacitor-forming insulating sheets 33, 34, 35,and interconnection insulating sheets (not shown), which are interposedbetween the respective insulating sheets 31, 32, 33 to 35 as required,are prepared. A ceramic green sheet such as a dielectric or the like ispreferably used as each of these insulating sheets.

Further, four coil conductors 16, 18 are formed in the coil-forminginsulating sheets 31, 32 in order to form the coils 11, 13 incorrespondence with the four signal wires 2, respectively. Further, theshapes of the respective coil conductors 16, 18 are preferably differentbetween the insulating sheets 31, 32 so that they are formed in a spiralconfiguration with respect to the laminating direction of the insulatingsheets 31, 32, respectively. Further, the directions of turns of therespective coil conductors 16, 18 are the same with respect to thedirection in which signals flow.

On the other hand, of the capacitor-forming insulating sheets 33, 34,35, the capacitance-forming electrodes 24, 25 are formed in the upperand lower insulating sheets 33, 35, respectively, and the shieldelectrode 23 is formed in the intermediate insulating sheet 34. A totalof four shield electrodes 23 and four capacitance-forming electrodes 24,25 are formed in parallel respectively in correspondence with the foursignal wires 2. Further, of the insulating sheets 31 to 35, the via hole20 or the like is formed in predetermined insulating sheets so as toprovide electrical connection between the upper and lower sheets.

A material such as Ag—Pd or Ag is preferably used for each of the coilconductors 16, 18, the shield electrode 23, and the capacitance-formingelectrodes 24, 25.

After laminating predetermined numbers of the output-side-coil-forminginsulating sheets 32, capacitor-forming insulating sheets 33 to 35, andinput-side-coil-forming insulating sheets 31, and, as required,interposing the interconnection insulating sheets (not shown) betweenthe respective insulating sheets 31 to 35, the laminate of theseinsulating sheets is integrally fired. Thereafter, on both side portions(outer left and right portions) of the insulator 4 thus obtained, theexternal electrodes 6, 7 are formed in correspondence with therespective signal wires 2.

Thus, the noise filter array according to the first preferred embodimenthaving the construction shown in FIG. 2 and the equivalent circuit shownin FIG. 3 is obtained. In the noise filter array according to the firstpreferred embodiment, the respective coil conductors 16, 18 aresequentially connected together through the via holes 17, 19, 20 tothereby form the spiral input-side coil 11 and the output-side coil 13,respectively. Further, the one end sides of the respective coils 11, 13are connected to the external electrodes 6, 7, and the other end sidesthereof are connected to each other in series through the via hole 20and are also commonly connected to the shield electrode 23. Further, thecapacitance-forming electrodes 24, 25 are opposed to the shieldelectrode 23 through the insulator 4 (insulator layer 4 a), and firstends of the respective capacitance-forming electrodes 24, 25 areconnected to the external electrodes 6, 7, thereby forming theinput-side capacitor 12 and the output-side capacitor 14, respectively.Accordingly, a construction is realized in which the input-sidecapacitor 12 and the output-side capacitor 14 are connected in parallelto the input-side coil 11 and the output-side coil 13, respectively.

When using the noise filter array according to the first preferredembodiment, in order to enable effective removal of noise in each of aplurality of frequency bands, the resonance point of each of theupstream and downstream LC parallel resonant circuits 8, 9 is preciouslyset to a resonance frequency at which noise included in each frequencyband is to be removed, whereby noise in two communication bands in thevicinity of 800 MHz and in the vicinity of 2 GHz, which are required asnoise countermeasures for potable telephones, for example, can beeffectively removed.

Further, since the shield electrode 23 is interposed between theinput-side coil 11 and the output-side coil 13, magnetic couplingbetween the upstream and downstream coils 11, 13 can be reliably cutoff. Accordingly, the resonance frequencies of the respective LCparallel resonant circuits 8, 9 do not vary, and the trap attenuation ineach of the plurality of frequency bands can be made large. For example,a high attenuation of about 20 dB or more can be secured for a resonancefrequency on the high frequency side.

Further, in the noise filter array according to the first preferredembodiment, a plurality of noise filters 3 are integrally disposedwithin a single component and can together remove noises in therespective signal wires 2. Accordingly, it is not necessary to provide anoise filter individually for each signal wire 2, thereby making itpossible to reduce the number of components. Further, since theplurality of LC parallel resonant circuits 8, 9 are disposed within thesingle insulator 4, it is possible to provide a highly reliable noisefilter array that is free from structural flaws with relatively littlefear of cracks or peels occurring during the manufacture thereof.

With respect to the noise filter array according to the first preferredembodiment of the present invention, the following evaluation experimentwas carried out in order to examine the filter characteristics thereof.

Evaluation Experiment

With respect to one of the noise filters 3 constituting the noise filterarray according to a preferred embodiment of the present invention, thefrequency-dependent characteristic (hereinafter, referred to as the “ILcharacteristic”) of the insertion loss (IL) thereof was examined. Here,the inductance L1 of the input-side coil 11 of the upstream LC parallelresonant circuit 8, the capacitance C1 of the input-side capacitor 12,the inductance L2 of the output-side coil 13 of the downstream LCparallel resonant circuit 9, and the capacitance C2 of the output-sidecapacitor 14 were set to approximately 24 nH, 1.2 pF, 18 nH, and 0.4 pF,respectively. FIG. 5 shows the results.

As shown in FIG. 5, it was confirmed that the noise filter 3 has aresonance frequency in each of two communication bands in the vicinityof 800 Hz and in the vicinity of the 2 GHz, which are required as noisecountermeasures for portable telephones, and thus can effectively removenoise included in each of the communication bands.

Second Preferred Embodiment

FIG. 6 is a sectional view of a noise filter array according to a secondpreferred embodiment of the present invention. In FIG. 6, the portionsthat are denoted by the same reference numerals as those of FIGS. 1 to 4indicate portions that are the same as or equivalent to those of thenoise filter array according to the first preferred embodiment.

The noise filter array according to the second preferred embodimentpreferably has the same equivalent circuit as that shown in FIG. 3.

Note that, however, that in the noise filter array according to secondpreferred embodiment, the input-side capacitor 12 and the output-sidecapacitor 13 are formed preferably by arranging the capacitance-formingelectrodes 24, 25 so as to be opposed to a portion of the coilconductors 16, 18 forming the input-side coil 11 and the output-sidecoil 13 through the insulator 4 (insulator layers 4 a), respectively.

That is, the input-side capacitor 12 is formed preferably by arrangingthe capacitance-forming electrodes 24 so as to be opposed to a portionof the coil conductors 16 on the output side forming the input-side coil11 through the insulator 4 (insulator layer 4 a), and the output-sidecapacitor 14 is formed preferably by arranging the capacitance-formingelectrode 25 so as to be opposed to a portion of the coil conductors 18on the input side forming the output-side coil 13 through the insulator4 (insulator layer 4 a). Further, first ends of the respectivecapacitance-forming electrodes 24, 25 are led out to the both outer sideportions of the insulator 4 to be electrically connected to the externalelectrodes 6, 7, respectively. Thus, two LC parallel resonant circuits8, 9 are provided, in which the input-side capacitor 12 and theoutput-side capacitor 14 are connected in parallel to the input-sidecoil 11 and the output-side coil 13, respectively.

Further, by adjusting the surface area or the distance over which thecapacitance-forming electrodes 24, 25 are opposed to the respective coilconductors 16, 18 to thereby vary the capacitances of the input-sidecapacitor 12 and output-side capacitor 14, the resonance points of therespective LC parallel resonant circuits 8, 9 are adjusted to resonancefrequencies at which noise is to be removed. The resonance points can bealso adjusted by adjusting the inductance of each of the input-sidecoils 11 and output-side coil 13.

Further, in the second preferred embodiment, as in the case of the firstpreferred embodiment, the shield electrode 23 is preferably providedbetween the input-side coil 11 on the upper side and the output-sidecoil 13 on the lower side, and is electrically connected to the via hole20 that provides serial connection between the input-side coil 11 on theupper side and the output-side coil 13 on the lower side.

When, as described above, the shield electrode 23 is provided betweenthe input-side coil 11 on the upper side and the output-side coil 13 onthe lower side, electromagnetic coupling between the upper and lowercoils 11, 13 is prevented even in cases where the component size issmall and the coils 11, 13 are in close proximity to each other, therebymaking it possible to secure high attenuation. However, depending on thecomponent size, there may be cases where a sufficient distance can besecured between the upper and lower coils 11, 13. In such cases, it ispossible to omit the shield electrode 23 because the magnetic couplingbetween the upper and lower coils 11, 13 becomes extremely small.

Further, while in the second preferred embodiment, the shield electrode23 is electrically connected to the via hole 20 that provides electricalconnection between the upper and lower coils 11, 13, in preventing themagnetic coupling between the upper and lower coils 11, 13, the shieldelectrode 23 may be electrically separated from the via hole 20.

Otherwise, the construction and effects of the second preferredembodiment are the same as those of the first preferred embodiment, sodetailed description is omitted here to avoid repetition.

Third Preferred Embodiment

FIG. 7 is a sectional view of a noise filter array according to thirdpreferred embodiment of the present invention. In FIG. 7, the portionsthat are denoted by the same reference numerals as those of FIGS. 1 to 4indicate portions that are the same as or equivalent to those of thenoise filter array according to the first preferred embodiment.

The noise filter array according to third preferred embodiment has thesame equivalent circuit as that shown in FIG. 3.

However, in the noise filter array according to third preferredembodiment, the input-side capacitor 12 and the output-side capacitor 14preferably include external electrodes 6, 7 for signal input/output,which are respectively formed on both end sides (outer left and rightportions) of the insulator 4 in correspondence with the respectivesignal wires 2, and the shield electrodes 23 that are commonly connectedto the upstream and downstream coils 11, 13 through the via hole 20.

That is, the input-side capacitor 12 preferably includes the input-sideexternal electrode 6, and the shield electrode 23 opposed to theinput-side external electrode 6 through the insulator 4 (4 b). Further,the output-side capacitor 14 preferably includes the output-sideexternal electrode 7, and the shield electrode 23 opposed to theoutput-side external electrode 7 through the insulator 4 (4 b). Therespective coils 11, 13 are thus connected in parallel to the capacitors12, 14, thereby defining the respective LC parallel resonant circuits 8,9.

Further, in the noise filter array according to the third preferredembodiment, by adjusting the distance over which each of the externalelectrodes 6, 7 and the shield electrode 23 are opposed to each other tothereby vary the capacitances of the input-side capacitor 12 andoutput-side capacitor 14, the resonance points of the respective LCparallel resonant circuits 8, 9 are adjusted to resonance frequencies atwhich noise is to be removed. The resonance points can be also adjustedby adjusting the inductance of each of the input-side coils 11 andoutput-side coil 13.

Otherwise, the construction and effects of third preferred embodimentare the same as those of first preferred embodiment, so detaileddescription is omitted here.

Fourth Preferred Embodiment

FIG. 8 is a sectional view of a noise filter array according to thefourth preferred embodiment of the present invention, FIG. 9 is anequivalent circuit diagram thereof, and FIG. 10 is an explodedperspective view showing a manufacturing method thereof.

In FIGS. 8 to 10, the portions that are denoted by the same referencenumerals as those of FIGS. 1 to 4 indicate portions that are the same asor equivalent to those of the noise filter array according to firstpreferred embodiment.

The noise filter array according to the fourth preferred embodiment has,in addition to the construction according to the first preferredembodiment described with reference to FIGS. 1 to 4, a construction inwhich the connecting points between the respective coils 11, 13 of theupstream and downstream LC parallel resonant circuits 8, 9, whichconstitute the noise filter 3 provided for each signal wire 2, arecommonly connected together in an ungrounded state via noise dispersingcapacitors 38.

That is, in the noise filter array, the input-side capacitor 12 and theoutput-side capacitor 14 are preferably formed by arranging thecapacitance-forming electrodes 24, 25 so as to be opposed to the shieldelectrode 23 provided inside the insulator 4, respectively. Further,noise dispersing electrodes 36 are arranged above and below the shieldelectrode 23 so as to be opposed to the shield electrode 23 through theinsulator 4 (insulator layers 4 a). The noise dispersing capacitor 38 ispreferably defined by the shield electrode 23 and each noise dispersingelectrode 36.

Further, in the noise filter array according to fourth preferredembodiment, while the shield electrode 23 and the capacitance-formingelectrodes 24, 25 are provided for respective signal wires 2 so as toextend along the signal wires 2, the upper and lower noise dispersingelectrodes 36 are formed continuously along the direction that issubstantially perpendicular to the respective signal wires 2 (thedirection that is substantially perpendicular to the plane of FIG. 8) soas to cross the respective electrodes 23, 24, 25. Further, the noisedispersing electrodes 36 are embedded in the insulator 4 so as to allowno external connection. That is, in addition to defining onecapacitance-forming electrode of each noise dispersing capacitor 38,each noise dispersing electrode 36 also defines the electrode forcommonly connecting the noise dispersing capacitors 38 to each other inan ungrounded state.

Next, a method of manufacturing the noise filter array according to thefourth preferred embodiment will be described with reference to FIG. 10.

Although the manufacturing method for the noise filter array accordingto the fourth preferred embodiment is basically the same as that in thefirst preferred embodiment, since it is necessary to form the noisedispersing capacitors 38 simultaneously with the formation of theupstream and downstream LC parallel resonant circuits 8, 9 for each ofthe four signal wires 2, the noise dispersing electrode 36 is formedsimultaneously in each of the insulating sheets 33, 35 in which therespective capacitance-forming electrodes 24, 25 are formed. Here, therespective noise dispersing electrodes 36 extend in a direction crossingthe capacitance-forming electrodes 24, 25 (the direction along thelongitudinal direction of the insulating sheets 33, 35).

Further, after laminating predetermined numbers of theoutput-side-coil-forming insulating sheets 32, capacitor-forminginsulating sheets 33 to 35, and input-side-coil-forming insulatingsheets 31, and, as required, interposing the interconnection insulatingsheets (not shown) between the respective insulating sheets 31 to 35,the laminate of these insulating sheets is integrally fired. Thereafter,on both side portions (outer left and right portions) of the insulator 4thus obtained, the external electrodes 6, 7 for signal input/output areformed in correspondence with the respective signal wires 2.

The noise filter array according to the fourth preferred embodiment isthus obtained, which has a construction in which, as shown in FIG. 8,the two upstream and downstream LC parallel resonant circuits 8, 9 areformed within the insulator 4 for each of the signal wires 2, and theconnecting points between the respective coils 11, 13 of the LC parallelresonant circuits 8, 9 are commonly connected together in an ungroundedstate via the noise dispersing capacitors 38, and which has theequivalent circuit as shown in FIG. 9.

In the noise filter array constructed as described above, a noisecurrent flowing in one signal wire 2 is reduced due to the loss in theLC parallel resonant circuits 8, 9 of each signal wire 2, and furtherdispersed to another signal wire 2 via the noise dispersing capacitor38. Therefore, when the above-described noise filter array is used,noise in each frequency band can be even more effectively removed thanin the first preferred embodiment. Further, since the noise filter has asharp cut off characteristic, the influence on the signal waveform canbe suppressed to be small.

Further, the electrode pattern for grounding, which is required in theprior art, becomes unnecessary, whereby an improvement can be achievedin terms of the freedom of the wiring layout of the circuit board 1(FIG. 1). Since it becomes possible to use a circuit board 1 having asimple construction, it is possible to achieve a reduction in cost.

Otherwise, the construction and effects of the fourth preferredembodiment are the same as those of the first preferred embodiment, sodetailed description is omitted here to avoid repetition.

While in the fourth preferred embodiment described above, the connectingpoints between the respective coils 11, 13 of the upstream anddownstream LC parallel resonant circuits 8, 9 are commonly connectedtogether in an ungrounded state via the noise dispersing capacitor 38,the present invention is not limited to this construction. For example,as shown in FIG. 11, a construction is also possible in which the noisedispersing capacitors 38 are commonly connected together in anungrounded state on the output sides of the downstream LC parallelresonant circuits 9. Alternatively, although not shown, a constructionis also possible in which, conversely, the noise dispersing capacitors38 are commonly connected together in an ungrounded state on the inputsides of the upstream LC parallel resonant circuits 8.

With respect to the noise filter array according to the fourth preferredof the present invention, the following evaluation experiment wascarried out in order to examine the filter characteristics thereof.

Evaluation Experiment 1

The IL characteristic of the noise filter array having the constructionaccording to the fourth preferred embodiment was examined. Here, inorder to prevent a difference in IL characteristic from occurring due tothe influence of cross talk, as in the equivalent circuit shown in FIG.12, measurement was carried out by connecting terminal resistors 37 of50Ω to the left and right ends of three of the four noise filters 3. Forthe purpose of comparison of characteristics, the IL characteristic wasexamined also with respect to the construction according to the firstpreferred embodiment with no noise dispersing capacitor 38 provided.Here, the measurement was carried out while uniformly setting theinductances of the input-side coils 11 of the upstream LC circuits 8 toabout 20 nH, the capacitances of the input-side capacitors 12 to about1.7 pF, the inductances of the output-side coils 13 of the downstream LCcircuits 9 to about 13 nH, and the capacitances of the output-sidecapacitors 14 to about 0.4 pF. FIG. 13 shows the results.

As shown in FIG. 13, in the case where the noise dispersing capacitors38 are provided, large signal attenuation was attained in the twocommunication bands in the vicinity of 800 MHz and in the vicinity of 2GHz, which are required as noise countermeasures for portabletelephones, and between the two bands. Thus, it was confirmed that noiseincluded in each of the communication bands can be effectively removed.

Evaluation Experiment 2

With respect to the noise filer array having the construction accordingto the fourth preferred embodiment, the IL characteristic in the casewhere the capacitance of the noise dispersing capacitor 38 is variedwithin the range of 0 pF to 15 pF was measured. In this case as well,the measurement was carried out by performing wiring connection so as torealize the equivalent circuit shown in FIG. 12. FIG. 14 shows theresults.

As shown in FIG. 14, it was confirmed that the larger the capacitance ofthe noise dispersing capacitor 38 is, the larger the attained signalattenuation is. However, when the capacitance becomes too large, crosstalk occurs in the signal frequency band, so the influence on the signalwaveform becomes large. In view of this, it is considered appropriate toset the capacitance of the noise dispersing capacitor 38 to 4 pF through10 pF.

Evaluation Experiment 3

The IL characteristic was measured with respect to the case (FIG. 9)where the connecting points between the respective coils 11, 13 of theupstream and downstream LC parallel resonant circuits 8, 9 are commonlyconnected together in an ungrounded state via the noise dispersingcapacitors 38, and the case (FIG. 11) where the noise dispersingcapacitors 38 are commonly connected together in an ungrounded state onthe output sides of the downstream LC parallel resonant circuits 9. Themeasurement conditions in this case were set to be the same as those forEvaluation Experiment 1. FIG. 15 shows the results. FIG. 15 also showsthe IL characteristic in the case where no noise dispersing capacitor 38is provided.

As shown in FIG. 15, in either cases of the construction shown in FIG. 9and the construction shown in FIG. 11, in comparison to the case whereno noise dispersing capacitor 38 is provided, a large signal attenuationwas attained in each of the two communication bands in the vicinity of800 MHz and in the vicinity of 2 GHz, which are required as noisecountermeasures for portable telephones. Thus, it was confirmed thatnoise included in each communication band can be effectively removed.

Incidentally, in each of the first through fourth preferred embodimentsdescribed above, the plurality of LC parallel resonant circuits 8, 9whose resonance frequencies are different from each other are formed byindividually connecting the capacitors 12, 14 in parallel to therespective coils 11, 13, respectively. Accordingly, the inductances ofthe coils 11, 13 and the capacitances of the capacitors 12, 14 can bereadily adjusted, whereby the resonance frequency of each of the LCparallel resonant circuits 8, 9 can be reliably set or controlled to adesired frequency required for the noise removal. Therefore, noiseremoval can be performed in a satisfactory manner for each of thefrequency bands.

On the other hand, the above-described advantages can be alsoaccomplished in a satisfactory manner by the constructions according tothe fifth, sixth and seventh preferred embodiments described below.

That is, as described above, in an LC parallel resonant circuit, theresonance frequency is dependent on the value of the LC product. As theLC product becomes larger, the resonance frequency becomes smallertoward the low frequency side. Further, provided that the value of theLC product is the same, the larger the inductance L, the larger theattenuation becomes, and the larger the ratio of the capacitance C, thenarrower the attenuation band becomes. Here, the setting of theresonance frequency on the high frequency side can be readily realizedby adjusting the floating capacitance because the LC product may besmall. Moreover, a wide attenuation band can be secured because a smallfloating capacitance suffices. On the other hand, for the setting of theresonance frequency on the low frequency side, the LC product must beset to be relatively large. In this case, since problems such asdistortion of the signal waveform occur when the value of the inductanceL is set too large, there is naturally a limit as to how large the valueof the inductance L can be set. Further, when, in order to compensatefor the limitations on the inductance L, the inter-layer distancebetween the coil conductors is reduced to set a large floatingcapacitance or the insulation material is changed, problems such as theloss of reliability due to degradation in characteristics or an increasein cost due to an increase in manufacturing man-hours occur.

In view of this, in the fifth to seventh preferred embodiments describedbelow, with respect to noise on the low frequency side, an LC parallelresonant circuit capable of providing a somewhat large LC product isdefined by the combination of a coil and a capacitor connected inparallel with the coil and, further, with respect to noise on the highfrequency side, an LC parallel resonant circuit having a required LCproduct is defined by a coil and a floating capacitance generatedbetween coil conductors (coil conductor layers) for forming the coil.Accordingly, a requisite noise removable action can be secured for eachfrequency band by means of a construction that is simpler than those ofEmbodiments 1 to 4. In the following, a more detailed description willbe given in this regard by way of Embodiments 5 to 7.

Fifth Preferred Embodiment

FIG. 16 is a sectional view of a noise filter array according to thefifth preferred embodiment of the present invention, FIG. 17 is anequivalent circuit diagram of the noise filter array according to thefifth preferred embodiment of the present invention, and FIG. 18 is anexploded perspective view showing a manufacturing method of the noisefilter array according to the fifth preferred embodiment of the presentinvention. In FIGS. 16 to 18, the portions that are denoted by the samereference numerals as those of FIGS. 1 to 4 indicate portions that arethe same as or equivalent to those of the noise filter array accordingto first preferred embodiment.

In the noise filter array according to the fifth preferred embodiment,an input-side capacitor 12 b is formed by arranging thecapacitance-forming electrode 24 so as to be opposed to a portion of theshield electrode 23 via the insulator 4 (insulator layer 4 a).

That is, the input-side capacitor 12 b is formed by arranging thecapacitance-forming electrode 24 so as to be opposed to a portion of theshield electrode 23, which is arranged between the upstream anddownstream coils 11, 13 so as to be substantially perpendicular to thecoil axis direction, through the insulator 4 (insulator layer 4 a).Further, one end of the capacitance-forming electrode 24 is led out toone end side (outer left side portion) of the insulator 4 to beelectrically connected to the external electrode 6. Accordingly, withrespect to the input-side coil 11, a floating capacitor 12 a (FIG. 17),which is naturally generated between the coil conductors (coil conductorlayers) 16 as a result of the formation of the coil 11, and theinput-side capacitor 12 b generated by the capacitance-forming electrode24 are both connected in parallel, thereby forming the LC parallelresonant circuit 8 on the low frequency side. Further, the LC parallelresonant circuit 9 on the high frequency side preferably includes theoutput-side coil 13, and a floating capacitor 14 a (FIG. 17) naturallygenerated between the coil conductors (coil conductor layers) 18 as aresult of the formation of the coil 13.

Further, in the noise filter array according to the fifth preferredembodiment, by adjusting the surface area or the distance over which thecapacitance-forming electrode 24 is opposed to each of the coilconductors 16 to thereby vary the capacitance of the input-sidecapacitor 12 b, the resonance point of the LC parallel resonant circuit8 on the low frequency side is adjusted to a resonance frequency atwhich noise is to be removed. It is also possible to adjust theresonance points of the respective LC parallel resonant circuits 8, 9 byadjusting the inductances of the input-side coil 11 and output-side coil13 or by adjusting the capacitances of the floating capacitors 12 a, 14a.

Further, in the fifth preferred embodiment, the shield electrode 23 isprovided between the input-side coil 11 on the upper side and theoutput-side coil 13 on the lower side, and is electrically connected tothe via hole 20 that provides serial connection between the input-sidecoil 11 on the upper side and the output-side coil 13 on the lower side.

As described above, according to the fifth preferred embodiment, in eachnoise filter 3, the LC parallel resonant circuit 8 on the upstream sideis constructed so as to have a requisite LC product through thecombination of the input-side coil 11, the floating capacitor 12 a, andthe input-side capacitor 12 b, noise on the low frequency side can beeffectively removed. That is, since a relatively large LC product can beset by the input-side coil 11 and the input-side capacitor 12 b, noiseon the low frequency side can be effectively removed while avoiding suchproblems that the signal waveform is distorted as the inductance L isset to an excessively large value, or a characteristic degradationoccurs or an increase in cost is caused by an increase in manufacturingman-hours because a large floating capacitor is set.

Further, the LC parallel resonant circuit 9 on the downstream sidepreferably includes the coil 13 and the floating capacitor 14 anaturally generated as a result of the formation of the coil 13, wherebynoise on the high frequency side can be effectively removed. That is,the setting of the resonance frequency on the high frequency side can bereadily realized by adjusting the capacitance of the floating capacitor14 a because a small LC product suffices. Further, the ratio of thefloating capacitance may be small, which proves advantageous because theattenuation band of noise on the high frequency side is not narrowed.For example, the communication bands for portable telephones are 875 MHzto 885 MHz on the low frequency side and 2110 MHz to 2170 MHz on thehigh frequency side, so the communication band on the high frequencyside is wider. According to the construction of the fifth preferredembodiment, noise on the high frequency side can be effectively removed.

Therefore, according to the fifth preferred embodiment, a requisitenoise removable action can be secured for each frequency band eventhrough the construction of each noise filter 3 is simpler than those ofthe first through fourth preferred embodiments.

Otherwise, the construction and effects of the fifth preferredembodiment are the same as those of the first preferred embodiment, sodetailed description is omitted here to avoid repetition.

Next, a method of manufacturing the noise filter array according to thefifth preferred embodiment will be described. Since the manufacturingmethod according to the fifth preferred embodiment is basically the sameas that of the first preferred embodiment, it will be described herebriefly.

For example, as shown in FIG. 18, predetermined numbers of theinput-side-coil-forming insulating sheets 31, output-side-coil-forminginsulating sheets 32, capacitor-forming insulating sheets 33, 34, andinterconnection insulating sheets (not shown) interposed between therespective insulating sheets 31, 32, 33, 34 as required, are prepared.

In this case, four coil conductors 16, 18 are disposed in thecoil-forming insulating sheets 31, 32 in order to provide the coils 11,13 in correspondence with the four signal wires 2, respectively.Further, of the capacitor-forming insulating sheets 33, 34, thecapacitance-forming electrode 24 is disposed in the insulating sheet 33on the upper side, and the shield electrode 23 is disposed in theinsulating sheet 34 on the lower side. Further, a total of four shieldelectrodes 23 and four capacitance-forming electrodes 24 are arranged inparallel in correspondence with the four signal wires 2. Further, of theinsulating sheets 31 to 34, the via hole 20 or the like is formed inpredetermined insulating sheets so as to provide electrical connectionbetween the upper and lower sheets.

Further, after laminating predetermined numbers of theoutput-side-coil-forming insulating sheets 32, capacitor-forminginsulating sheets 33, 34, and input-side-coil-forming insulating sheets31, and, as required, interposing the interconnection insulating sheets(not shown) between the respective insulating sheets 31 to 34, thelaminate of these insulating sheets is integrally fired.

Thereafter, on both side portions (outer left and right portions) of theinsulator 4 thus obtained, the external electrodes 6, 7 are formed incorrespondence with the respective signal wires 2. Thus, the noisefilter array according to the fifth preferred embodiment having theconstruction as shown in FIG. 16 and having and equivalent circuit asshown in FIG. 17 is obtained.

Sixth Preferred Embodiment

FIG. 19 is a sectional view of a noise filter array according to thesixth preferred embodiment of the present invention. In FIG. 19, theportions that are denoted by the same reference numerals as those ofFIGS. 1 to 4 indicate portions that are the same as or equivalent tothose of the noise filter array according to the first preferredembodiment.

The noise filter array according to the sixth preferred embodimentpreferably has the same equivalent circuit as that shown in FIG. 17.

However, in the noise filter array according to the sixth preferredembodiment, the input-side capacitor 12 b is preferably formed byarranging the capacitance-forming electrodes 24 so as to be opposed to aportion of the coil conductors 16 forming the input-side coil 11 throughthe insulator 4 (insulator layers 4 a).

That is, the input-side capacitor 12 b is formed by arranging thecapacitance-forming electrodes 24 so as to be opposed to a portion ofthe output side of the coil conductors 16 forming the input-side coil 11through the insulator 4 (insulator layers 4 a). Further, one end of eachof the capacitance-forming electrodes 24 is led out to one end side(outer left side portion) of the insulator 4 to be electricallyconnected to the external electrode 6.

Accordingly, with respect to the input-side coil 11, the floatingcapacitor 12 a (see FIG. 17), which is naturally generated between thecoil conductors (coil conductor layers) 16 as a result of the formationof the coil 11, and the input-side capacitor 12 b generated by thecapacitance-forming electrodes 24 are both connected in parallel,thereby forming the LC parallel resonant circuit 8 on the low frequencyside. Further, the LC parallel resonant circuit 9 on the high frequencyside includes the input-side coil 13, and the floating capacitor 14 a(see FIG. 17) naturally generated between the coil conductors (coilconductor layers) 18 as a result of the formation of the coil 13.

Further, in the noise filter array according to the sixth preferredembodiment, by adjusting the surface area or the distance over which thecapacitance-forming electrode 24 is opposed to each of the coilconductors 16 to thereby vary the capacitance of the input-sidecapacitor 12 b, the resonance point of the LC parallel resonant circuit8 on the low frequency side is adjusted to a resonance frequency atwhich noise is to be removed. It is also possible to adjust theresonance points of the respective LC parallel resonant circuits 8, 9 byadjusting the inductances of the input-side coil 11 and output-side coil13 or by adjusting the capacitances of the floating capacitors 12 a, 14a.

Further, in the sixth preferred embodiment, as in the fifth preferredembodiment, the shield electrode 23 is preferably provided between theinput-side coil 11 on the upper side and the output-side coil 13 on thelower side, and is electrically connected to the via hole 20 thatprovides serial connection between the input-side coil 11 on the upperside and the output-side coil 13 on the lower side.

Otherwise, the construction and effects of the sixth preferredembodiment are the same as those of first preferred embodiment, sodetailed description is omitted here to avoid repetition.

Seventh Preferred Embodiment

FIG. 20 is a sectional view of a noise filter array according to aseventh preferred embodiment of the present invention. In FIG. 20, theportions that are denoted by the same reference numerals as those ofFIGS. 1 to 4 indicate portions that are the same as or equivalent tothose of the noise filter array according to the first preferredembodiment.

The noise filter array according to the seventh preferred embodiment hasthe same equivalent circuit as that shown in FIG. 17.

However, in the noise filter array according to the seventh preferredembodiment, the input-side capacitor 12 b preferably includes theexternal electrode 6 for signal input/output, which is formed on one endside (outer left side portion) of the insulator 4 in correspondence witheach of the signal wires 2, and the shield electrodes 23 located betweenthe upstream and downstream coils 11, 13, the shield electrodes 23 beingsubstantially perpendicular to the coil axis direction and electricallyconnected to the coils 11, 13 through via hole 20.

That is, the input-side capacitor 12 b preferably includes theinput-side external electrode 6, and the shield electrodes 23 opposed tothe input-side external electrode 6 through the insulator 4 (insulatorlayers 4 a). Accordingly, with respect to the input-side coil 11, thefloating capacitor 12 a (see FIG. 17), which is naturally generatedbetween the coil conductors (coil conductor layers) 16 as a result ofthe formation of the coil 11, and the input-side capacitor 12 bgenerated by the shield electrodes 23 are both connected in parallel,thereby forming the LC parallel resonant circuit 8 on the low frequencyside. Further, the LC parallel resonant circuit 9 on the high frequencyside preferably includes the output-side coil 13, and the floatingcapacitor 14 a (see FIG. 17) naturally generated between the coilconductors (coil conductor layers) 18 following the formation of thecoil 13.

Further, in the noise filter array according to the seventh preferredembodiment, by adjusting the distance over which the respective externalelectrodes 6 and the shield electrodes 23 are opposed to each other orthe like to thereby vary the capacitance of the input-side capacitor 12b, the resonance point of the LC parallel resonant circuit 8 on the lowfrequency side is adjusted to a resonance frequency at which noise is tobe removed. It is also possible to adjust the resonance points of therespective LC parallel resonant circuits 8, 9 by adjusting theinductances of the input-side coil 11 and output-side coil 13 or byadjusting the capacitances of the floating capacitors 12 a, 14 a.

Otherwise, the construction and effects of the seventh preferredembodiment are the same as those of the fifth preferred embodiment, sodetailed description is omitted here to avoid repetition.

While in each of various preferred embodiments described above, thedescription is directed to the case where the two upstream anddownstream LC parallel resonant circuits 8, 9 are provided with respectto the respective signal wires 2, the present invention is not limitedto this. It is also possible to adopt a construction in which three ormore LC parallel resonant circuits are connected in tandem with respectto the respective signal wires 2. In that case, noise removingcharacteristics over an even wider band can be attained by setting theinductance L and the capacitance C as appropriate for each LC parallelresonant circuit so that an appropriate resonance frequency is obtained.

Further, while in each of the above-described various preferredembodiments, the description is directed to the case where the fournoise filters 3 are combined in correspondence with the four signalwires 2 located on the circuit board 1 and integrated into a noisefilter array, the number of the noise filters 3 is not particularlylimited. The present invention is also applicable to cases where asingle noise filter 3 is provided. Further, while in the fourthpreferred embodiment as well the description is directed to the casewhere the four noise filters 3 are combined in correspondence with thefour signal wires 2 and integrated into a noise filter array, in thiscase, too, the number of signal wires 2 or the number of noise filters 3is not particularly limited.

In other respects as well, the present invention is not restricted tothe preferred embodiments of the present invention described above butcan be subject to various applications and modifications within thescope of the present invention.

According to the present invention, it is possible to provide a noisefilter which makes it possible to easily and reliably set a resonancefrequency in each of a plurality of frequency bands and which is capableof efficiently removing noise in each of the plurality of frequencybands, and also, a noise filter which makes it possible to attain a highattenuation at each resonance frequency by reliably preventing magneticcoupling between the coils, and a noise filter array including such anoise filter.

The noise filter and the noise filter array according to variouspreferred embodiments of the present invention can be suitably used forapplications such as removal of noise in portable telephones, andfurther can be used for a wide variety of other applications (forexample, applications such as removal of noise in other high-frequencycircuits).

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A noise filter for removing noise flowing in a signal wire located ona circuit board, the noise filter comprising: an insulator; a pair ofexternal electrodes that are connected to the signal wire and aredisposed on an outside of the insulator; a plurality of coils connectedin series, disposed inside of the insulator, and having both endsthereof electrically connected to the external electrodes, respectively;and a capacitor connected in parallel to at least one of the pluralityof coils; wherein each of the plurality of coils includes a plurality ofcoil conductors disposed in the insulator, the plurality of coilconductors being arranged in a spiral configuration and connected toeach other through a via hole; and the capacitor includes a shieldelectrode and a capacitance-forming electrode opposed to each otherthrough the insulator, the shield electrode being located betweenupstream and downstream coils of the plurality of coils and commonlyelectrically connected to both the upstream and downstream coils, thecapacitance-forming electrode being electrically connected to one of thepair of external electrodes.
 2. The noise filter according to claim 1,wherein the shield electrode has a surface area that is equal to orgreater than about ½ of a surface area of a bore of at least one of theupstream and downstream coils.
 3. The noise filter according to claim 1,wherein a plurality of LC parallel resonant circuits having differentresonance frequencies are defined by the plurality of coils and thecapacitor individually connected in parallel to each of the plurality ofcoils.
 4. The noise filter according to claim 1, wherein an LC parallelresonant circuit on a low frequency side is defined by one of theplurality of coils, the capacitor connected in parallel to the one ofthe plurality of coils, and a floating capacitor generated due to thepresence of the one of the plurality of coils, and an LC parallelresonant circuit on a high frequency side is defined by another one ofthe plurality of coils and a floating capacitor generated due to thepresence of the another one of the plurality of coils.
 5. A noise filterarray comprising a plurality of the noise filters according to claim 1,wherein the plurality of the noise filters are integrated together whilebeing arranged in an array individually in correspondence with aplurality of signal wires located on a circuit board.
 6. The noisefilter array according to claim 5, wherein connecting points between theplurality of coils provided for each of the signal wires are commonlyconnected together in an ungrounded state via a noise dispersingcapacitor.
 7. A noise filter for removing noise flowing in a signal wirelocated on a circuit board, the noise filter comprising: an insulator; apair of external electrodes that are connected to the signal wire andare disposed on an outside of the insulator; a plurality of coilsconnected in series, disposed inside of the insulator, and having bothends thereof electrically connected to the external electrodes,respectively; and a capacitor connected in parallel to at least one ofthe plurality of coils; wherein each of the plurality of coils includesa plurality of coil conductors disposed in the insulator, the pluralityof coil conductors being arranged in a spiral configuration andconnected to each other through a via hole; and the capacitor includesthe plurality of coil conductors and a capacitance-forming electrodearranged so as to be opposed to each other through the insulator, thecapacitance-forming electrode being electrically connected to one of thepair of external electrodes.
 8. The noise filter according to claim 7,wherein a shield electrode is disposed between upstream and downstreamcoils of the plurality of coils so as to be substantially perpendicularto a coil axis direction of the plurality of coils.
 9. The noise filteraccording to claim 8, wherein the shield electrode has a surface areathat is equal to or greater than about ½ of a surface area of a bore ofat least one of the upstream and downstream coils.
 10. The noise filteraccording to claim 7, wherein a plurality of LC parallel resonantcircuits having different resonance frequencies are defined by theplurality of coils and the capacitor individually connected in parallelto each of the plurality of coils.
 11. The noise filter according toclaim 7, wherein an LC parallel resonant circuit on a low frequency sideis defined by one of the plurality of coils, the capacitor connected inparallel to the one of the plurality of coils, and a floating capacitorgenerated due to the presence of the one of the plurality of coils, andan LC parallel resonant circuit on a high frequency side is defined byanother one of the plurality of coils and a floating capacitor generateddue to the presence of the another one of the plurality of coils.
 12. Anoise filter array comprising a plurality of the noise filters accordingto claim 7, wherein the plurality of the noise filters are integratedtogether while being arranged in an array individually in correspondencewith a plurality of signal wires located on a circuit board.
 13. Thenoise filter array according to claim 12, wherein connecting pointsbetween the plurality of coils provided for each of the signal wires arecommonly connected together in an ungrounded state via a noisedispersing capacitor.
 14. A noise filter for removing noise flowing in asignal wire located on a circuit board, the noise filter comprising: aninsulator; a pair of external electrodes that are connected to thesignal wire and are disposed on an outside of the insulator; a pluralityof coils connected in series, disposed inside of the insulator, andhaving both ends thereof electrically connected to the externalelectrodes, respectively; and a capacitor connected in parallel to atleast one of the plurality of coils; wherein each of the plurality ofcoils includes a plurality of coil conductors disposed in the insulator,the plurality of coil conductors being arranged in a spiralconfiguration and connected to each other through a via hole; and thecapacitor includes one of the pair of external electrodes and a shieldelectrode arranged so as to be opposed to each other through theinsulator, the shield electrode being located between upstream anddownstream coils of the plurality of coils and electrically connected toboth the upstream and downstream coils.
 15. The noise filter accordingto claim 14, wherein the shield electrode has a surface area that isequal to or greater than about ½ of a surface area of a bore of at leastone of the upstream and downstream coils.
 16. The noise filter accordingto claim 14, wherein a plurality of LC parallel resonant circuits havingdifferent resonance frequencies are defined by the plurality of coilsand the capacitor individually connected in parallel to each of theplurality of coils.
 17. The noise filter according to claim 14, whereinan LC parallel resonant circuit on a low frequency side is defined byone of the plurality of coils, the capacitor connected in parallel tothe one of the plurality of coils, and a floating capacitor generateddue to the presence of the one of the plurality of coils, and an LCparallel resonant circuit on a high frequency side is defined by anotherone of the plurality of coils and a floating capacitor generated due tothe presence of the another one of the plurality of coils.
 18. A noisefilter array comprising a plurality of the noise filters according toclaim 14, wherein the plurality of the noise filters are integratedtogether while being arranged in an array individually in correspondencewith a plurality of signal wires located on a circuit board.
 19. Thenoise filter array according to claim 18, wherein connecting pointsbetween the plurality of coils provided for each of the signal wires arecommonly connected together in an ungrounded state via a noisedispersing capacitor.